The present invention relates to a photolithography reticle for use in manufacturing semiconductor devices, and more particularly to a reticle and method of making it, which can be used to sharpen the light used to expose a masking material and thereby improve the definition of fabricated features.
Photolithography is commonly used in semiconductor device fabrication to pattern various photomasks used in processing a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and template such as a mask or reticle. During the exposure process, radiant energy, such as ultraviolet light, is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, forming thereafter a resist mask on the wafer. The resist mask can be used further to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
An integral component of the photolithographic process is the reticle. The reticle includes the pattern for passing, blocking, or phase shifting light to expose the photoresist which is used to form features (e.g., transistor or capacitor structures) at a particular layer of a semiconductor device. The reticle is typically a quartz plate coated with a patterned light blocking material such as, for example, chromium. This type of mask is typically called a binary mask because light is completely blocked by the light blocking material and fully transmitted through the unblocked quartz portions.
Binary masks pose various problems when fabricating dense circuits. Light passing through the edge of a pattern within the mask (e.g., the boundary between a light blocking region and a transparent region) is oftentimes diffracted. This means that instead of producing a very sharp image of the edge on the resist layer, some lower intensity light diffracts beyond the intended edge boundary and into the regions expected to remain dark. Hence, the resultant feature shapes and sizes deviate somewhat from the intended IC design. Because integrated circuit manufacturers have continued to reduce the geometric size of the IC features, this diffraction can produce wafers having dies with incomplete or erroneous circuit patterns.
Attenuated phase shift masks (PSMs) have been used to overcome the diffraction effects and to improve the resolution and depth of focus of images projected onto a target (i.e., the photoresist). Attenuated PSMs utilize partially transmissive regions in addition to the light blocking and light transmissive regions used in binary masks. The partially transmissive regions typically pass (i.e., do not block) about three to eight percent of the light they receive. Moreover, the partially transmissive regions are designed so that the light they do pass is shifted by 180 degrees in comparison to the light passing through the transparent (e.g., transmissive) regions. Thus, some of the light spreading outside of the transparent region defined by the PSM pattern edge destructively interferes with light passing from the partially transmissive regions. This way, the detrimental effects caused by diffraction can be controlled.
As it is known in the art, reticle layouts are generated for each photoresist layer that must be patterned. Each reticle layout includes a pattern for blocking/passing light which is designed to produce, through the exposed photoresist, corresponding circuit features. One such representative reticle is shown in FIG. 1. This reticle is a 0-180 degree attenuated phase shift reticle used to produce patterned areas for etching an insulated layer, e.g. a BPSG layer, to produce wells for fabricating container capacitors. Portions of an original reticle layout have been modified by OPC (optical proximity correction) techniques to generate a modified layout so that if exposure were directed through a reticle having such a modified layout, the photoresist would be exposed in a pattern which includes features which more closely approximates the corresponding desired feature in a circuit layout. The modified layout may be generated using any known algorithms or by other techniques, for instance, using trial and error through experience with particular layouts.
An attenuated phase shift mask which has different regions of differing phase shift values may be made in a variety of ways. For example, the 0-180 degree phase shift mask, illustrated in FIG. 1, can be made by taking a substrate of a transparent material, such as quartz, having a thickness such that incident light passing through the layer is of the same phase as the light entering the layer (0 degree phase shift), and etching into the side of the quartz layer where light exits to a depth which will shift the phase of incident exposure by 180 degrees (relative to the 0 degree regions, i.e., the regions of the layer which are not etched) to produce the 180 degree phase shift regions. A chrome layer is also applied on the light entering side to block these portions of the quartz substrate where incident light should not pass through the substrate and a layer of partially transmissive material is also applied on the quartz layer at the light entry side, over the etched regions, to form the partially transmissive 180 degree phase shift regions.
This way, transmissive or transparent regions 12 of open quartz, the partially transmissive regions 14, and the blocked regions combine to form a light pattern in a photoresist layer. The transparent regions 12 pass the light without a phase shift. The partially transmissive regions 14 pass only about six percent of the light they receive with a 180 degree phase shift. The material used to form the partially transmissive regions 14 is any suitable opaque material, for example, molybdenum silicide (Moxe2x80x94Si) or chromium fluoride. A preferred material for use in making the transparent regions 12 of reticle 10 is quartz. However, any other suitable light transmissive material such as soda-lime glass, borosilicate glass, or other similar natural or synthetic materials can be used also. Light blocking regions are typically formed with a chrome layer on the quartz substrate.
Although the FIG. 1 attenuated phase shift reticle is adequate for many applications, as semiconductors sizes continue to decrease, the light pattern produced by it becomes an increasing problem. FIG. 2 is the aerial image response with a critical dimension contour (CD) of the printing image of a capacitor design formed with the attenuated phase shift reticle layout of FIG. 1. FIG. 2 depicts different regions which correspond to different light intensities which are produced by the FIG. 1 reticle. The contour of the desired printing image is delineated by line 21. When exposed in a positive photoresist, the area encapsulated within line 21 of FIG. 2, that is the area including zones 23, 25, and 27, is removed and an etch opening is formed in a photoresist. Unfortunately, the light intensity contours of adjacent areas in FIG. 2, such as regions 23 and 25 for example, are not sharply defined since at sub-micron levels, light is diffracted and affected by proximity effects. Accordingly, there is a blurring of light, or stated otherwise, a light transition region across the boundaries of the defined intensity regions 23, 25, and 27.
Proximity effects occur primarily when very closely spaced circuit pattern features are lithographically transferred to a resist layer on a wafer. The light waves of the closely spaced circuit features interact, thereby distorting the final transferred pattern features. Accordingly, features that are in close proximity to other features tend to be more significantly distorted than features which are relatively isolated from other features.
As a consequence of the unsharp profiles in light intensity from one region to the next, the edges of the developed photoresist pattern tend to be less well defined in these areas than in other areas of the masked pattern. In small, dense integrated circuits, such as VLSI, these blurred images can cause printing of features which may significantly degrade a circuit""s performance, since the correspondence between the actual circuit design and the final transferred circuit pattern on the photoresist layer is decreased. Further, unsharp profiles can result in a loss of wafer surface area, which correspondingly reduces the total area available for deposited conductors and accordingly results in undesirable increase in contact resistance.
Accordingly, there is a need for a simplified phase shift reticle which can be used to precisely fabricate small circuit features, for example, closely spaced wells for container capacitors used in a memory circuit.
The present invention provides an alternating phase shift mask for a capacitor layout scheme for a memory device integrated circuit. The alternating phase shift mask has regions of 0 and 180 degree phase shifts arranged in a way such that all sides of each region corresponding to a given phase shift value are bounded by areas corresponding to an opposite phase shift value.
The present invention also provides a method for producing an alternating phase shift reticle having regions of 0 and 180 degree phase shifts arranged in a way such that all sides of each region corresponding to a given phase shift value are bounded by areas corresponding to an opposite phase shift value. The reticle can be used to produce densely packed capacitor features, in which the variance between the actual exposure pattern and the desired exposure pattern is reduced. The alternating phase shift reticle of the present invention counteracts the diffraction and proximal effects, while improving both the resolution and depth of focus of the transmitted light.
Additional advantages and features of the present invention will become more readily apparent from the following detailed description of the invention, which is provided in connection with accompanying drawings.